Sperm oxidative status varies with the level of sperm competition and affects male reproductive success

Abstract

Sperm competition theory predicts that males should strategically adjust the allocation of sperm in response to the level of sperm competition (i.e. the number of competing ejaculates). Predictions of sperm competition models have mostly focused on sperm numbers. However, whether males strategically adjust other important traits of their ejaculate such as sperm antioxidants in response to potential rivals and the consequences of such allocation strategies on sperm integrity (e.g. oxidative damage accumulation) and offspring viability is poorly investigated. Using an experimental design in which I manipulated the perceived level of sperm competition experienced by male field crickets, Gryllus bimaculatus , I show that besides sperm numbers, sperm oxidative status can also vary in response to the level of sperm competition. Male crickets increased the antioxidant content of their ejaculates in response to sperm competition risk, an effect that presumably led to less sperm oxidative damage, at least when the level of competition was low. Moreover, I provide evidence suggesting that such strategic adjustments can have fitness consequences for males, as eggs sired by males with increased sperm damage had lower viability. Overall, my results suggest that sperm competition drives plasticity in nonsperm components affecting sperm oxidative status and support the idea that oxidative stress may play a key role in postcopulatory sexual selection. • Male crickets adjusted sperm numbers as expected by sperm competition theory. • Males also increased sperm antioxidants in response to other potential rivals. • Males under a low level of sperm competition had reduced sperm oxidative damage. • Eggs fertilized by males with reduced sperm oxidative damage were more viable.